1. Field of the Invention
The present invention relates to a polyvinyl alcohol-based hollow fiber membrane and a process for producing the same. More specifically, the present invention relates to a polyvinyl alcohol-based hollow fiber membrane having a sharp fractionating capability, with which the ratio between the size of particles of 90% rejection and that of particles of 10% rejection is not more than 5, and to a process for producing the same.
2. Description of the Prior Art
Hollow fiber membranes from polyvinyl alcohol (herein-after referred to as PVA), which is representative of hydrophilic polymers, are widely put in actual uses as various separation membranes. PVA-based hollow fiber membranes are produced by, for example, a process which comprises extruding an aqueous solution of a vinyl alcohol-based polymer into an aqueous solution of a dehydrating agent such as sodium sulfate, a process which comprises extruding an aqueous solution of a vinyl alcohol-based polymer into an aqueous solution of an alkali such as sodium hydroxide and a process which comprises extruding an aqueous solution of a vinyl alcohol-based polymer containing boric acid or a borate into an aqueous alkaline solution of a dehydrating salt, such as an aqueous mixed solution of sodium hydroxide and sodium sulfate. See, for example, Japanese Patent Publication Nos. 15268/1979 and 40654/1979.
There has been in recent years an increasing demand for a separation membrane having sharp fractionating capability that can separate substances having only slightly different particle sizes. Intensive studies have been made to obtain a PVA-based hollow fiber membrane meeting this demand. However, it is difficult to obtain, by the above disclosed processes, a PVA-based hollow fiber membrane that is satisfactory in the fractionating capability. At present, membranes with unsatisfactory fractionating capability are therefore being used as they are. On the other hand, the objects to be separated by membrane separation become more sophisticated year by year, thereby requiring a membrane having sharper fractionating capability.